1. Field of the Invention
The present invention relates to a system having a waste heat recovery subsystem having a heat exchanger for extracting and recovering waste heat from the waste heat medium by means of a transfer fluid, and in particular, relates to methods and apparatus for control of such heat recover systems.
2. Description of the Prior Art
With the present day emphasis on energy conservation, a number of various types of apparatus for use with refrigeration and air conditioning equipment have been developed in which the waste heat from the condensers can be recovered and used for various purposes. For example, many homes now utilize the waste heat from home air conditioners for heating water for domestic use with significant savings in energy and costs. An example of such art is U.S. Pat. No. 3,922,876 assigned to the assignee of the instant application, entitled "Energy Conservation Unit." This patent discloses heat recovery system which extracts heat from the superheated fluorocarbon refrigerant in an air conditioning system by passing water through a heat exchanger installed in the air conditioner refrigerant line to the condenser. In addition to supplying essentially free hot water, the additional cooling of the refrigerant increases the efficiency of the air conditioner, resulting in lower primary power consumption.
This system and similar prior art systems have been controlled, with respect to the flow of water through the heat exchanger, by a combination of thermostatically controlled valves and switches, and by the use of electrochemical relays. For example, it is necessary to control a pump located in the water line of the recovery system to move the supply of cold water through the heat exchanger and back to the water storage tank only during operational periods of the air conditioner. When the water reaches a pre-selected upper temperature, which may be selected on the basis of safety or the temperature required for a particular application, thermostatically contolled valves have been utilized to shut off the flow of water. In addition, switches and relays have been necessary to turn the pump off and on at the appropriate times.
These prior art control systems have a number of obvious disadvantages. Accurate thermostats which operate mechanically are relatively expensive and increase the cost of home installations. Low cost thermostats have been found to vary widely in their characteristics and may result in water either too hot or too cold for the intended use.
Another problem that has developed in such units stems from the necessity of having the heat exchanger in close proximity to the condenser, which is usually located in an outside environment. During cold weather, water in the lines to the heat transfer unit may freeze, resulting in serious damage to the system.
Many prior arty systems also require connections to the air conditioner electrical control circuits to operate the relays which prevent the water circulation through the heat exchanger except when the air conditioner is in operation. This approach, of course, has the disadvantage of requiring relays and requiring modification or connection to the air conditioner electrical system. Alternatively, other systems have depending upon a thermostat located at the outlet of the heat exchanger to control the pump. Thus, when the air conditioner first begins to operate, the pump will be off and there is no water flow through the heat exchanger. Initially, as the water in the heat exchanger becomes heated from the flow of hot refrigerant, there is no circulation of water taking place. Since the system depends upon heat reaching the external thermostat by conduction, water in the heat exchanger itself may reach a very high temperature and exceed a safe limit before the pump is activated.